John Cusack wastes no time getting down to business. "I've made 10 good films," says the 41-year-old actor shortly after striding into the Berlin hotel room in a backwards baseball cap and sprawling in an armchair. "I'm sure you know which ones they are. The ones that suck I tend to blank out. It's like I never even made them." I'm slightly taken aback at his honesty, though his tally is in the right ballpark. Here's my list: The Sure Thing, Eight Men Out, Say Anything, The Grifters, Bullets Over Broadway, Grosse Pointe Blank, The Thin Red Line, Being John Malkovich, High Fidelity, Max - that pretty much covers it. But has he really made 40 movies that suck? He mulls it over. "Well, there aren't 40 that are great, put it that way." He pauses for an eternity, eyes widening. "But that's fine. Ten is a good batting average, don't you think?"

To that hallowed 10 he now adds his latest picture, a claustrophobic chiller called 1408. What a happy coincidence, you might think: the movie he happens to be promoting this week is one about which he is cock-a-hoop. But it doesn't take long in Cusack's company to realise he's a straight-shooter. Isn't that why audiences respond to him? Whether he's playing chirpy and idealistic (Say Anything) or amoral (The Grifters, Grosse Pointe Blank), he cuts through the celebrity fog that surrounds so many performers of his stature and generation; you feel you're getting something real, even if another of his selling-points is paradoxically that you can never be sure of its precise nature. "This John Cusack guy: I always see something going on in there, and I don't know what it is," remarked the late Robert Altman. And if Altman couldn't define it, what hope the rest of us?

Not only has Cusack learned a lot about the way the industry works in his 20-plus years as an actor, but he is eager to share his findings; and to shatter any illusions I might have about how an actor such as him comes to make the films he does - only 20% of which, don't forget, are actually any cop. So it's unlikely he would beat around the bush if he had reservations about 1408. It is adapted from a Stephen King story (Cusack's second: he was the hero's idealized older brother in Stand by Me) about a hack who checks into a malevolent hotel room as research for a book on haunted hostelries. What he encounters there is a radio clock that only plays the Carpenters, eerie paintings that spill free of their frames and a hammer-wielding assailant with a scary receding hairline.

He falters somewhat talking about the finished film. "If I'm in something that I think is kinda good, it stays with me like a fever dream for a long time afterwards. I don't recall the finished product so much as the feeling of making it." Working largely on his own altered the typical film-making dynamic: if being on a movie set is like living in a bubble, then Cusack was in a bubble within that bubble. "It was all so intimate," he enthuses. "The director, the cinematographer and I created our own little world, with its rules and internal logic that only we understood. I loved it because it was a high-wire act. I knew that if it worked, it would be great, and if it didn't, I'd fall on my ass real hard. It's insane doing something where you don't know whether it's going to work on any level, but it's so exciting." His long face crinkles into a grin.

Only a handful of actors have ever been entrusted with the lion's share of an entire film - think of Tom Hanks shooting the breeze with a basketball for most of Cast Away, or Philip Baker Hall prowling the Oval Office alone as Nixon in Secret Honour. But to this elite breed we can now add Cusack, who dominates 1408. Samuel L Jackson is there to hand over the key, and various bit-players drift in and out of Cusack's ensuing hallucinations, but for the most part we are alone with him and his distinctive brand of rumpled, smart-aleck cynicism.

When I ask him about how consciously he has cultivated this persona, he gives an easy-going shrug. "I suppose I have a certain thing I do well that people seem to like. Not everyone likes it, of course. The guy in the Guardian last week certainly didn't." Cusack is referring to a negative review of 1408, in which Joe Queenan wrote that the actor's "wise-cracking slacker persona is starting to wear thin". He seems both amused and quietly irritated by this remark. "So there you go," he smiles. "Some people like it, other people don't." Yes, I insist, but what exactly is it that you do well? He's laughing now. "Well, you see I'm trying to avoid answering that question."

With films like The Sure Thing, Say Anything and High Fidelity, Cusack developed a knowing, slightly nerdy screen image that was a forerunner of what Seth Rogen is flogging in Knocked Up. In these movies, Cusack became a symbol of hope, both for those men who figured it might not be so bad being a nerd after all, and for those women who found themselves dating one. "I'm aware of the affection those characters inspired," he says. "I feel close to Lloyd in Say Anything. He was like a super-interesting version of me. Only I'm not as good as him. Whatever part of me is romantic and optimistic, I reached into that to play Lloyd. Of course, now it's all gone. Now I'm just bitter."

There seems to be some discomfort involved for Cusack in articulating what he's good at, but he gamely has a go anyway. "I think I'm pretty brave," he says seriously. "I'll take risks. I can look at my career and point to the movies that were risky." He singles out Max, in which he played a fictional gallery-owner who urges the young Adolf Hitler to channel his rage into his painting, and Being John Malkovich, where his down-at-heel puppeteer charged customers to enter Malkovich's head via a clammy hole behind some filing cabinets. "Being John Malkovich worked out great, so people tend to forget what a risk it was - first-time writer, first-time director and so on. I read that screenplay four years before it got made. I'd said to my agents: 'Show me scripts that are fantastic and crazy.' I love getting up on that tightrope. I wish I could do it more, but I have to balance what I want to do with what people want me to do."

Who are these "people"? "People who offer me work," he says. "There's this brand that they think I am, and I get sent stuff that corresponds to that. I have to do it. It's not like there are 10 projects on offer at any one time, and six of them are brilliant." I'm astonished that Cusack hasn't earned the right to pick and choose, given his track record. "It's absolutely true," he says. "No one cares. The movies have got more corporate, they're making fewer movies in general, and those they are making are all $200-$300m tent-pole releases that eat up all the oxygen."

This may seem rich coming from a man who profited from exactly that species of movie when he appeared as an FBI agent in the Jerry Bruckheimer-produced blockbuster Con Air. Yes, his character wore sandals and quoted Dostoevsky, but he still ended the film wrestling the bad guy atop a speeding fire engine. At the time, Cusack claimed he did Con Air because it had come time for him to be a businessman. "You know: get my name above the title, my face on a billboard," he told me in 1997. "I use those kinds of films to get leverage," he says now. "You wouldn't think Con Air had anything to do with Max, but in my career it does. It's doing Con Air, or doing romantic comedies, that makes Max possible. The bad stuff you just try to make as good as you can."

It may be pragmatic, but isn't it also depressing? "Sure, it's depressing." Another long, wide-eyed pause. "But you aren't gonna talk about that in the press. 'Poor John, he's depressed because he can't have it all his way' - you know, with everything going on in the world that's going to sound ridiculous." He hoots at the thought. "I get to do the stuff I want. I have a good voice, I think, and it comes through in my work."

To illustrate this, he cites two upcoming films concerned in varying degrees with war: Grace Is Gone, in which he plays a man whose wife is killed in Iraq, and War, Inc, a "spiritual cousin to Grosse Pointe Blank", and partly inspired by Naomi Klein's article Baghdad Year Zero.

You can see the strategy in juggling such disparate projects, but from the outside it resembles a kind of personality crisis. When the same actor who throws himself into writing, producing and starring in War, Inc or Grosse Pointe Blank turns up in featherweight romantic daydreams such as Serendipity or Must Love Dogs, it's as though there are two John Cusacks walking the earth.

To his credit, he retains a palpable sense of mystery on screen and off; the most diligent showbiz reporter would be hard-pressed to fill a paragraph about him. "The thing about John," says Mikael Hafstrom, the director of 1408, "is that he is full of secrets. You never read anything about him in the gossip papers, he doesn't talk about his private life, so you never feel you've had enough of him." Cusack calls it his survival instinct. "It seems like common sense to me. When I was growing up, I never wanted to know what my favourite musicians or artists ate for breakfast, or who they were dating. I found out what they felt about the world from their work."

On the downside, there is the real possibility that he will get taken for granted - that he'll always be there, being wry and enigmatic, and is destined never to receive proper approbation. He was daring in The Grifters and Being John Malkovich, and droll in Bullets Over Broadway, and yet on all those occasions he had to stand by as virtually everyone associated with those films, from the caterer upwards, was nominated for Academy Awards while he - the leading man, no less, in all three cases - was overlooked. Doesn't he want an award? "Do you have one?" he shoots back, mock-excitedly. "If you wanna give me an award, I'll take it. Just don't make me go to the party afterwards."